Thylacosmilus Atrox
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''Thylacosmilus'' is an extinct
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of saber-toothed
metatheria Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as wel ...
n
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s that inhabited
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...
from the
Late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million ye ...
to
Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58epochs In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...
. Though ''Thylacosmilus'' looks similar to the "
saber-toothed cats Machairodontinae (from Ancient Greek μάχαιρα ''Makhaira, machaira,'' a type of Ancient Greek sword and ὀδόντος ''odontos'' meaning tooth) is an extinct subfamily of carnivoran mammals of the cat family Felidae, representing the ...
", it was not a
felid Felidae ( ) is the Family (biology), family of mammals in the Order (biology), order Carnivora colloquially referred to as cats. A member of this family is also called a felid ( ). The 41 extant taxon, extant Felidae species exhibit the gre ...
, like the well-known American ''
Smilodon ''Smilodon'' is an extinct genus of Felidae, felids. It is one of the best known saber-toothed predators and prehistoric mammals. Although commonly known as the saber-toothed tiger, it was not closely related to the tiger or other modern cats ...
'', but a
sparassodont Sparassodonta (from Greek to tear, rend; and , gen. , ' tooth) is an extinct order of carnivorous metatherian mammals native to South America, related to modern marsupials. They were once considered to be true marsupials, but are now thought ...
, a group closely related to
marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...
s, and only superficially resembled other saber-toothed mammals due to
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
. A 2005 study found that the bite forces of ''Thylacosmilus'' and ''Smilodon'' were low, which indicates that the killing techniques of saber-toothed animals differed from those of extant species. Remains of ''Thylacosmilus'' have been found primarily in Catamarca, Entre Ríos, and
La Pampa Province La Pampa () is a sparsely populated province of Argentina, located in the Pampas in the center of the country. Neighboring provinces are from the north clockwise San Luis, Córdoba, Buenos Aires, Río Negro, Neuquén and Mendoza. History In ...
s in northern
Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...
.


Taxonomy

In 1926, the Marshall Field Paleontological Expeditions collected mammal fossils from the
Ituzaingó Formation The Ituzaingó Formation (), in older literature also described as Entre Ríos or Entrerriana Formation, is an extensive formation (geology), geological formation of Late Miocene (Tortonian, or Huayquerian in the South American land mammal age, SA ...
of Corral Quemado, in
Catamarca Province Catamarca () is a province of Argentina, located in the northwest of the country. The province had a population of 429,556 as per the , and covers an area of 102,602 km2. Its literacy rate is 95.5%. Neighbouring provinces are (clockwise, f ...
, northern Argentina. Three specimens were recognized as representing a new type of
marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...
, related to the
borhyaenids Borhyaenidae is an extinct metatherian family of low-slung, heavily built predatory mammals in the order Sparassodonta. Borhyaenids are not true marsupials, but members of a sister taxon, Sparassodonta. Like most metatherians, borhyaenids and oth ...
, and were reported to the Paleontological Society of America in 1928, though without being named. In 1933, the American paleontologist Elmer S. Riggs named and preliminarily described the new genus ''Thylacosmilus'' based on these specimens, while noting that a full description was being prepared and would be published at a later date. He named two new species in the genus, ''T. atrox'' and ''T. lentis''. The generic name ''Thylacosmilus'' means "pouch knife", while the specific name ''atrox'' means "cruel". Riggs found the genus distinct enough to warrant a new
subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end botanical subfamily names with "-oideae", and zo ...
within Borhyaenidae, Thylacosmilinae, and stated it was "one of the most unique flesh-eating mammals of all times". The
holotype specimen A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was Species description, formally described. It is either the single such physical example (or illus ...
of ''T. atrox'', FMNH P 14531, was collected by Riggs and an assistant. It consists of a skull with the teeth of the right side entirely preserved as well as the left canine found separate in the
matrix Matrix (: matrices or matrixes) or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the m ...
, fragments of the mandibles, and a partial skeleton consisting of a humerus, a broken radius and broken femora, and foot bones. Missing and scattered parts of the skull and mandible were reconstructed and fitted together. Specimen P 14344 was designated as the
paratype In zoology and botany, a paratype is a specimen of an organism that helps define what the scientific name of a species and other taxon actually represents, but it is not the holotype (and in botany is also neither an isotype (biology), isotype ...
of ''T. atrox'', and consists of the skull, the mandible, seven cervical, two dorsal, two lumbar, and two sacral vertebrae, a femur, a tibia, a fibula, and various foot bones. It was one fourth smaller than the holotype, and may have been a young adult. It was collected by the American paleontologist Robert C. Thorne. The holotype of ''T. lentis'', specimen P 14474, is a partial skull with the teeth of the right side preserved, and is about the same size as the ''T. atrox'' paratype. It was collected a few miles away from the site of the ''T. atrox'' holotype discovery, by the German biologist
Rudolf Stahlecker Rudolf Stahlecker (25 November 1898 in Sternenfels near Pforzheim – 26 October 1977 in Urach) was a German geologist and biology teacher. Biography He studied with the German paleontologist Friedrich von Huene at the University of Tüb ...
. These specimens were housed at the
Field Museum of Natural History The Field Museum of Natural History (FMNH), also known as The Field Museum, is a natural history museum in Chicago, Illinois, and is one of the largest such museums in the world. The museum is popular for the size and quality of its educationa ...
in
Chicago Chicago is the List of municipalities in Illinois, most populous city in the U.S. state of Illinois and in the Midwestern United States. With a population of 2,746,388, as of the 2020 United States census, 2020 census, it is the List of Unite ...
, while the ''T. lentis'' type later became part of the Museum of La Plata collection. In 1934, Riggs fully described the animal, after the fossils had been prepared and compared with other mammals from the same formation and better known borhyaenids from the
Santa Cruz Formation The Santa Cruz Formation is a geological formation in the Magallanes Basin, Magallanes/Austral Basin in southern Patagonia in Argentina and adjacent areas of Chile. It dates to the late Early Miocene epoch, and is contemporaneous with the eponym ...
. More fragmentary ''Thylacosmilus'' specimens have since been discovered. Riggs and the American paleontologist
Bryan Patterson Bryan Patterson (born 10 March 1909 in London; died 1 December 1979 in Chicago) was an American paleontologist at the Field Museum of Natural History in Chicago. Life and career Bryan Patterson was the son of the soldier, engineer and author ...
reported in 1939 that a canine (MLP 31-XI-12-4) tentatively assigned to ''Achlysictis'' or '' Stylocynus'' by the Argentinian paleontologist Lucas Kraglievich in 1934 belonged to ''Thylacosmilus''. A partial right ramus and front half of a skull (MLP 65_VI 1-29-41.) was collected in 1965. In a 1972 thesis, the Argentinian paleontologist Jorge Zetti suggested that ''T. atrox'' and ''T. lentis'' represented a single species, and the American paleontologist Larry G, Marshall agreed in 1976, stating the features distinguishing the two were of dubious taxonomic value, and probably due to differences in age and sex. He also found it hard to explain how two sympatric species (related species that lived in the same area at the same time) would be virtually identical in their specializations. Marshall also suggested '' Hyaenodonops'' could be cogeneric, though it was impossible to determine from the available specimens.


Evolution

Though ''Thylacosmilus'' is one of several predatory mammal genera typically called "
saber-toothed cat Machairodontinae (from Ancient Greek μάχαιρα '' machaira,'' a type of Ancient Greek sword and ὀδόντος ''odontos'' meaning tooth) is an extinct subfamily of carnivoran mammals of the cat family Felidae, representing the earliest ...
s", it was not a
felid Felidae ( ) is the Family (biology), family of mammals in the Order (biology), order Carnivora colloquially referred to as cats. A member of this family is also called a felid ( ). The 41 extant taxon, extant Felidae species exhibit the gre ...
placentalia Placental mammals ( infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distingui ...
n, but a
sparassodont Sparassodonta (from Greek to tear, rend; and , gen. , ' tooth) is an extinct order of carnivorous metatherian mammals native to South America, related to modern marsupials. They were once considered to be true marsupials, but are now thought ...
, a group closely related to
marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...
s, and only superficially resembled other saber-toothed mammals due to
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
. The term "saber-tooth" refers to an
ecomorph Ecomorphology or ecological morphology is the study of the relationship between the ecological role of an individual and its morphological adaptations. The term "morphological" here is in the anatomical context. Both the morphology and ecology ex ...
consisting of various groups of extinct predatory
synapsid Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...
s (mammals and close relatives), which
convergently evolved Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
extremely long
maxillary canine In human dentistry, the maxillary canine is the tooth located laterally (away from the midline of the face) from both maxillary lateral incisors of the mouth but mesial (toward the midline of the face) from both maxillary first premolars. Both the ...
s, as well as adaptations to the skull and skeleton related to their use. This includes members of
Gorgonopsia Gorgonopsia (from the Greek Gorgon, a mythological beast, and 'aspect') is an extinct clade of Saber-toothed predator, sabre-toothed therapsids from the Middle Permian, Middle to the Upper Permian, roughly between 270 and 252 million years ago. ...
,
Thylacosmilidae Thylacosmilidae is an extinct family of metatherian predators, related to the modern marsupials, which lived in South America between the Miocene and Pliocene epochs. Like other South American mammalian predators that lived prior to the Great Ame ...
,
Machaeroidinae Machaeroidinae ("dagger-like") is an extinct subfamily of carnivorous saber-toothed placental mammals, from the extinct family Oxyaenidae, that lived from the early to middle Eocene of Asia and North America.Malcolm C. McKenna, Susan K. Bell: ' ...
,
Nimravidae Nimravidae is an extinct family (biology), family of carnivorans, sometimes known as false saber-toothed cats, whose fossils are found in North America and Eurasia. Not considered to belong to the true cats (family Felidae), the nimravids are gen ...
,
Barbourofelidae Barbourofelidae (or Barbourofelinae) is an extinct family (biology), family (or subfamily) of carnivorans of the suborder Feliformia, sometimes known as false saber-toothed cats, that lived in North America, Eurasia and Africa during the Miocene ...
, and
Machairodontinae Machairodontinae (from Ancient Greek μάχαιρα ''Makhaira, machaira,'' a type of Ancient Greek sword and ὀδόντος ''odontos'' meaning tooth) is an extinct subfamily of carnivoran mammals of the cat family Felidae, representing the ...
. The
cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
below shows the position of ''Thylacosmilus'' within Sparassodonta, according to Suarez and colleagues, 2015.


Description

Body mass for sparassodonts is difficult to estimate, since these animals have relatively large heads in proportion to their bodies, leading to overestimations, particularly when compared with skulls of modern members of Carnivora, which have different locomotive and functional adaptations, or with those of the recent predatory marsupials, which do not exceed of body mass. Recent methods, like Ercoli and Prevosti's (2011) linear regressions on postcranial elements that directly support the body's weight (such as tibiae, humeri and ulnae), comparing ''Thylacosmilus'' to both extinct and modern carnivorans and metatherians, suggest that it weighed between , with Sorkin 2008 suggesting an estimate to , about the same size as a modern
jaguar The jaguar (''Panthera onca'') is a large felidae, cat species and the only extant taxon, living member of the genus ''Panthera'' that is native to the Americas. With a body length of up to and a weight of up to , it is the biggest cat spe ...
. Sanz-Pérez ''et al.'' 2024 study suggested that ''Thylacosmilus'' was much smaller than the previous estimates, weighing about . The differences in weight estimations may be due to the individual size variation of the specimens studied in each analysis, as well as the different samples and methods used. In any case, the weight estimations are consistent for terrestrial species that are generalists or have some degree of cursoriality. A weight in this range would make ''Thylacosmilus'' one of the largest known
carnivorous A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose nutrition and energy requirements are met by consumption of animal tissues (mainly mu ...
metatherians.


Skull

A 2025 study found that ''T. atrox'' showed lower
encephalization quotient Encephalization quotient (EQ), encephalization level (EL), or just encephalization is a relative brain size measure that is defined as the ratio between observed and predicted brain mass for an animal of a given size, based on nonlinear regre ...
compared to hathliacynids, but higher than borhyaenoids. The
endocast An endocast is the internal cast of a hollow object, often referring to the cranial vault in the study of brain development in humans and other organisms. Endocasts can be artificially made for examining the properties of a hollow, inaccessible ...
of ''T. atrox'' was gyrencephalic, globular, and inclined, in contrast to that of most other sparassodonts, whose brains were broadly similar to those of extant marsupials. ''Thylacosmilus'' had large, saber-like canines. The roots of these canines grew throughout the animal's life, growing in an arc up the maxilla and above the orbits. ''Thylacosmilus'' teeth are in many aspects even more specialized than the teeth of other sabertoothed predators. In these animals the predatory function of the "sabres" gave rise to a specialization of the general dentition, in which some teeth were reduced or lost. In ''Thylacosmilus'' the canines are relatively longer and more slender, relatively triangular in cross-section, in contrast with the oval shape of carnivorans' saber-like canines. The function of these large canines was once thought to have apparently even eliminated the need for functional incisors, while carnivorans like ''
Smilodon ''Smilodon'' is an extinct genus of Felidae, felids. It is one of the best known saber-toothed predators and prehistoric mammals. Although commonly known as the saber-toothed tiger, it was not closely related to the tiger or other modern cats ...
'' and ''
Barbourofelis ''Barbourofelis'' is an extinct genus of large, predatory, from a subfamily of feliformia, feliform carnivoran mammals of known as Barbourofelidae, Barbourofelinae, part of the Nimravidae superfamily. ''Barbourofelis'', along with ''Albanosmilus' ...
'' still have a full set of incisors. However, evidence in the form of wear facets on the internal sides of the lower canines of ''Thylacosmilus'' indicate that the animal did indeed have incisors, though they remain hitherto unknown due to poor fossilization and the fact that no specimen thus far has been preserved with its premaxilla intact. In ''Thylacosmilus'' there is also evidence of the reduction of postcanine teeth, which developed only a tearing cusp, as a continuation of the general trend observed in other sparassodonts, which lost many of the grinding surfaces in the premolars and molars. The canines were hypsodont and more anchored in the skull, with more than half of the tooth contained within the
alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit. Uses in anatomy and zoology * Pulmonary alveolus, an air sac in the lungs ** Alveolar cell or pneumocyte ** Alveolar duct ** Alveolar macrophage * M ...
, which were extended over the braincase. They were protected by the large symphyseal flange and they were powered by the highly developed musculature of the neck, which allowed forceful downward and backward movements of the head. The canines had only a thin layer of enamel, just 0.25 mm in its maximum depth at the lateral facets, this depth being consistent down the length of the teeth. The teeth had open roots and grew constantly, which eroded the abrasion marks that are present in the surface of the enamel of other sabertooths, such as ''Smilodon''. The sharp serrations of the canines were maintained by the action of the wear with the lower canines, a process known as thegosis. The convex upper portion of the maxilla is ornamented with extensive furrows and pits. This texturing has been correlated with an extensive network of blood vessels, which may suggest that the upper maxilla was covered by some form of soft tissue which tentatively has been hypothesized as a "horn covering" (keratinous structure).


Postcranial skeleton

Although the postcranial remains of'' Thylacosmilus ''are incomplete, the elements recovered so far allow the examination of characteristics that this animal acquired in convergence with the sabertooth felids. Its cervical vertebrae were very strong and to some extent resembled the vertebrae of
Machairodontinae Machairodontinae (from Ancient Greek μάχαιρα ''Makhaira, machaira,'' a type of Ancient Greek sword and ὀδόντος ''odontos'' meaning tooth) is an extinct subfamily of carnivoran mammals of the cat family Felidae, representing the ...
; also the cervical vertebrae have neural apophysis well developed, along with ventral apophysis in some cervicals, an element that is characteristic of other borhyaenoids. The lumbar vertebrae are short and more rigid than in '' Prothylacynus''. The bones of the limbs, like the
humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...
and
femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...
, are very robust, since they probably had to deal with larger forces than in the modern felids. In particular, the features of the humerus suggest a great development of the pectoral and deltoid muscles, not only required to capture its prey, also to absorb the energy of the impact of the collision with such prey. The features of the hindlimb, with a robust femur equipped with a
greater trochanter The greater trochanter of the femur is a large, irregular, quadrilateral eminence and a part of the skeletal system. It is directed lateral and medially and slightly posterior. In the adult it is about 2–4 cm lower than the femoral head.Sta ...
in the lower part, the short
tibia The tibia (; : tibiae or tibias), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two Leg bones, bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outsi ...
and plantigrade feet shows that this animal was not a runner, and probably stalked its prey animals. The hindlimbs also allowed a certain mobility of the hip, and possibly the ability to stand up only with its hindlimbs, like ''Prothylacynus'' and ''
Borhyaena ''Borhyaena'' is an extinct genus of South American metatherian, living between 17.5 and 15.5 million years ago in Patagonia, Argentina ( Santa Cruz and Sarmiento Formations) and Chile ( Río Frias Formation).barbourofelids and
nimravid Nimravidae is an extinct family of carnivorans, sometimes known as false saber-toothed cats, whose fossils are found in North America and Eurasia. Not considered to belong to the true cats (family Felidae), the nimravids are generally considered ...
s, the claws of ''Thylacosmilus'' were not retractable.


Palaeobiology


Diet and feeding

Recent comparative biomechanical analysis have estimated the bite force of ''T. atrox'', starting from maximum gape, at , much weaker than that of a
leopard The leopard (''Panthera pardus'') is one of the five extant cat species in the genus ''Panthera''. It has a pale yellowish to dark golden fur with dark spots grouped in rosettes. Its body is slender and muscular reaching a length of with a ...
, suggesting its jaw muscles had an insignificant role in the dispatch of prey. Its skull was similar to that of ''
Smilodon ''Smilodon'' is an extinct genus of Felidae, felids. It is one of the best known saber-toothed predators and prehistoric mammals. Although commonly known as the saber-toothed tiger, it was not closely related to the tiger or other modern cats ...
'' in that it was much better adapted to withstand loads applied by the neck musculature, which, along with evidence for powerful and flexible forelimb musculature and other skeleton adaptations for stability, support the hypothesis that its killing method consisted on immobilization of its prey followed by precisely directed, deep bites into the soft tissue driven by powerful neck muscles. It has been suggested that its specialized predatory lifestyle could be linked to more extensive parental care than in modern marsupial predators, due that the killing technique only could be used by adult individuals with a full development of its peculiar dental anatomy and grasping abilities; it could require some time for young individuals to learn the necessary skills, although there are no clear evidence in the fossils ''Thylacosmilus'', and this kind of cooperative behavior is unknown in modern marsupials. In 1988 Juan C. Quiroga published a study on the
cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. It is the largest site of Neuron, neural integration in the central nervous system, and plays ...
of two proterotherids and ''Thylacosmilus''. The study examines endocranial casts of two ''Thylacosmilus'' specimens: MLP 35-X-41-1 (from the
Montehermosan The Montehermosan age is a period of geologic time (6.8–4.0 Mya (unit), Ma) within the Miocene and Pliocene epochs of the Neogene used more specifically with South American Land Mammal Ages. It follows the Huayquerian and precedes the Chapadmalal ...
age in
Catamarca Province Catamarca () is a province of Argentina, located in the northwest of the country. The province had a population of 429,556 as per the , and covers an area of 102,602 km2. Its literacy rate is 95.5%. Neighbouring provinces are (clockwise, f ...
), which represents a natural cast of the left half of the cranial cavity lacking the anterior part of the olfactory bulbs and the brain hemispheres; and MMP 1443 (from the
Chapadmalalan The Chapadmalalan age is a period of geologic time (4.0–3.0 Ma) within the Pliocene epoch of the Neogene used more specifically with South American Land Mammal Ages. It follows the Montehermosan and precedes the Uquian age. Fossil content ...
age in
Buenos Aires Province Buenos Aires, officially the Buenos Aires Province, is the largest and most populous Provinces of Argentina, Argentine province. It takes its name from the city of Buenos Aires, the capital of the country, which used to be part of the province an ...
), which is a complete, artificial cast that shows some ventral displacement but with the anterior right part of the brain hemisphere and olfactory bulb. Quiroga's analysis showed that the
somatic Somatic may refer to: * Somatic (biology), referring to the cells of the body in contrast to the germ line cells ** Somatic cell, a non-gametic cell in a multicellular organism * Somatic nervous system, the portion of the vertebrate nervous syst ...
nervous system of ''Thylacosmilus'' represented 27% of the entire cortex, with the visual area representing 18% and the auditory area 7%. The
paleocortex In anatomy of animals, the paleocortex, or paleopallium, is a region within the telencephalon in the vertebrate brain. This type of cerebral cortex, cortical tissue consists of three cortical laminae (layers of perikaryon, neuronal cell bodies). I ...
was more than 8%. The sulci of the cortex are relatively complex and similar in pattern and number to the modern
diprotodont Diprotodontia (, from Greek "two forward teeth") is the largest extant order of marsupials, with about 155 species, including the kangaroos, wallabies, possums, koala, wombats, and many others. Extinct diprotodonts include the hippopotamus-sized ...
marsupials. Compared with ''
Macropus ''Macropus'', from the Ancient Greek words μάκρος (''mákros''), meaning "long", and πους (''pous''), meaning "foot", is a marsupial genus in the family Macropodidae. It has two extant species of large terrestrial kangaroos. Thirteen kn ...
'' and ''
Trichosurus The brushtail possums are the members of the genus ''Trichosurus'' in the Phalangeridae, a family of marsupials. They are native to Australia (including Tasmania) and some small nearby islands. Unique among marsupials, they have shifted the hyp ...
'', ''Thylacosmilus'' had less development of the maxillar area with respect to the mandibular area, and the rhinal fissure is taller than in ''Macropus'' and ''
Thylacinus ''Thylacinus'' is a genus of extinct carnivorous marsupials in the family Thylacinidae. The only recent member was the thylacine The thylacine (; binomial name ''Thylacinus cynocephalus''), also commonly known as the Tasmanian tiger or ...
''. This disproportion between the maxillar and mandibular areas, which are roughly similar in marsupials, seems to be a consequence of the extreme development of the neck and mandibular musculature, used in the functioning of the osteodentary anatomy of this animal. However, the area dedicated to the oral-mandible region comprises 42% of the somatic area. The comparison between the endocranial casts of ''Thylacosmilus'' and a proterotherid specimen (possibly a species coevolving with ''Thylacosmilus'' and a potential prey item) indicates that ''Thylacosmilus'' had only half of the encephalization and a quarter of the
cortical area The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. It is the largest site of Neuron, neural integration in the central nervous system, and plays ...
, however it has more somatized areas, similar visual areas and less auditory area, which suggests different sensomotoric qualities between both animals. The analysis published by Christine Argot in 2002 about the evolution of predatory borhyaenoids suggests that ''Thylacosmilus'' was a specialized form, which have a limited
stereoscopic vision Binocular vision is seeing with two eyes, which increases the size of the visual field. If the visual fields of the two eyes overlap, binocular depth can be seen. This allows objects to be recognized more quickly, camouflage to be detected, spa ...
with small eyes, with an overlap of 50-60°, very low compared with modern predators, but the ossified and great auditory bulla and the muscular body would indicate that it could be an ambush predator in open and relatively dry environments, where the sound absorption is lesser than in more humid areas, and the acute hearing could compensate the limited vision. Argot suggested that ''Thylacosmilus'' maybe was a nocturnal hunter, as modern lions. Studies published in 2023 by Gaillard ''et al.'' suggest that despite the unique placement and divergences of the eyes, ''Thylacosmilus'' was still granted some stereoscopic visual capability as a result of the frontation and verticality of its eye orbits, with this adaptation being a trade-off as a result of the unique morphology of its teeth, which never stopped growing. This study also suggests that ''Thylacosmilus'' was largely unimpeded in predatory capability by the reduction in binocular vision created by its hypertrophied canines. A 2005 study published by Wroe ''et al''. analysing bite-forces with the use of regressions on body mass and applying the model of "dry skull" in which the jaw is modeled as a lever based in relations between the skull dimensions and the jaw muscles, was employed in some extinct and extant placental and metatherian predatory mammals. ''Thylacosmilus atrox'' reached the lowest value in that analysis, just barely surpassed by ''Smilodon fatalis''. The authors concluded that both taxa, with low bite forces and peculiar cranial and postcranial anatomies, had a killing technique to dispatch large bodied prey without a true analogue between modern taxa. An analysis by Goswami ''et al''. in 2010 tested if the metatherian mode of reproduction has produced any constraint in their cranial morphological evolution. Using landmarks in the skulls of several
eutherian Eutheria (from Greek , 'good, right' and , 'beast'; ), also called Pan-Placentalia, is the clade consisting of placental mammals and all therian mammals that are more closely related to placentals than to marsupials. Eutherians are distingu ...
and metatherian meat-eating lineages, they compared the ecomorphological convergences in these groups. Metatherian lineages, including specialised forms as ''
Thylacoleo ''Thylacoleo'' ("pouch lion") is an extinct genus of carnivorous marsupials that lived in Australia from the late Pliocene to the Late Pleistocene (until around 40,000 years ago), often known as marsupial lions. They were the largest and last mem ...
'' and ''Thylacosmilus,'' showed values in morphospace more similar to
caniniform In mammalian oral anatomy, the canine teeth, also called cuspids, dogteeth, eye teeth, vampire teeth, or fangs, are the relatively long, pointed tooth, teeth. In the context of the upper jaw, they are also known as ''fangs''. They can appear mo ...
s than felids, due that even the shortening of the skull and reduction of postcanine teeth are not so drastic as in felids, despite them often being compared to feliform eutherians. The study shows that in any case, metatherians could be so diverse in cranial diversity as its eutherian counterparts, even with very extreme forms as ''Thylacosmilus'' itself and that the metatherian development doesn't have any significative role in cranial evolution. A 2020 study found several functional disparities between ''Thylacosmiluss cranial anatomy and that of saber-toothed eutherians that cannot be explained by its metatherian status, such as the lack of a jaw symphysis, subtriangular canines instead of blade-like ones, lack of incisors (that would render feline-like feeding behaviours impossible), weak jaw musculature and unaligned teeth with no evidence of shearing activity, as well as a post-cranial skeleton more akin to that of a bear than a cursorial predator like a cat. This study very tentatively suggests that ''Thylacosmilus'' might have been an intestine specialist that slashed open and sucked up the carcass' entrails. A 2021 statistical analysis conversely concluded that ''Thylacosmilus'' killed in the same manner as other sabre-tooths, because the premaxillary area, the carnassial region, and the nape of ''Smilodon'', ''Homotherium'', ''Barbourofelis'', and ''Thylacosmilus'' are all similarly developed, which they presumed was to, respectively, withstand high bite forces, maximise gape, and strengthen neck-driven head pulling. ''Thylacosmilus'' scored closest to ''Barbourofelis''. A 2020 isotope ratio study, which used
stable isotopes The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundan ...
of carbon and oxygen from the
tooth enamel Tooth enamel is one of the four major Tissue (biology), tissues that make up the tooth in humans and many animals, including some species of fish. It makes up the normally visible part of the tooth, covering the Crown (tooth), crown. The other ...
of several mammals from the Pampean region from the Late Miocene to Late Pleistocene, was published by Domingo ''et al''. in 2020 and indicates that the favoured prey of ''Thylacosmilus'' were grazers, mainly
notoungulates Notoungulata is an extinct order of ungulates that inhabited South America from the early Paleocene to the end of the Pleistocene, living from approximately 61 million to 11,000 years ago. Notoungulates were morphologically diverse, with forms re ...
from open areas. This diet seems to coincide with the expansion of vast grasslands of C4 plants in southern South America and the increasing of aridity and lower temperatures, in the interval between 11 million and 3 million years ago known as ''Edad de las Planicies Australes'' ("Age of the Southern Plains", in Spanish). A more recent isotope study by Sanz-Pérez ''et al.'' suggests ''Thylacosmilus'' hunted prey in more open environments than the smaller ''
Lycopsis ''Lycopsis'' is an extinct genus of South American metatherian that lived during the Miocene in Argentina and Colombia. History Although not named until 1927, Florentino Ameghino described a species now seen as synonymous with ''Lycopsis torre ...
'', preying on large mammals with the lowest ''δ''13C values, likely hunted in small-medium sized notoungulates and
litopterns Litopterna (from "smooth heel") is an extinction, extinct order of South American native ungulates that lived from the Paleocene to the Pleistocene-Holocene around 62.5 million to 12,000 years ago (or possibly as late as 3,500 years ago), and we ...
, and as well as large
rodents Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...
.


Motion

Various studies have been published on the musculature and motion of ''Thylacosmilus''. The analysis made by William Turnbull published in 1976 and 1978 included a reconstruction of the masticatory muscles of ''Thylacosmilus'' modelling them with
plasticine Plasticine is a putty-like modelling material made from calcium salts, petroleum jelly and aliphatic acids. Though originally a brand name for the British version of the product, it is now applied generically in English as a product category ...
over a cast of the skull and following the muscle scars on the surface of the fossil, then making a rubber model of the musculature and calculating the percentage of weight of these muscles compared with recent mammals, he concluded that the muscles involved in jaw closing in this animal were not unusual in size neither in form, compared with modern carnivore mammals, even indicating that they weren't so reduced as in the machairodont felids. Turnbull concluded that in ''Thylacosmilus'' these masticatory muscles was not involved at all in the use of the sabertooth canines, which depended of the large neck muscles and the flexion of the head to be used killing the prey, combining in a sense the stabbing and slashing techniques from " dirk-toothed" and "
scimitar A scimitar ( or ) is a single-edged sword with a convex curved blade of about 75 to 90 cm (30 to 36 inches) associated with Middle Eastern, South Asian, or North African cultures. A European term, ''scimitar'' does not refer to one specific swor ...
" sabertooths. The comparative studies of Argot 2004, indicates that the
basicranium The base of skull, also known as the cranial base or the cranial floor, is the most inferior area of the skull. It is composed of the endocranium and the lower parts of the calvaria. Structure Structures found at the base of the skull are for ...
had rugose crests that served as attachments for the neck flexor muscles, which are associated to the increase of the bite strength. The deltopectoral crest is large, with 60% of the length of the humerus, which is correlated with musculature to manipulate heavy prey. This animal had an absent entepicondylar foramen in the humerus, which is correlated with the reduction of the abduction movement in that bone in cursorial ungulates and carnivores (''
Borhyaena ''Borhyaena'' is an extinct genus of South American metatherian, living between 17.5 and 15.5 million years ago in Patagonia, Argentina ( Santa Cruz and Sarmiento Formations) and Chile ( Río Frias Formation).adductor muscles. Although the lumbar vertebrae are not completely known, the two last ones are known and suggest for its vertical neural process that there is not an anticlinal vertebra; probably the muscles of the back (''m. longissimus dorsi'') acted to stabilize the column and contribute to body propulsion, as occurs in ''Smilodon'', which contrast with more flexible backs of the closest relatives of these sabertooth taxa.


Distribution and habitat

Based on studies of its habitat, ''Thylacosmilus'' is believed to have hunted in savanna-like or sparsely forested areas, avoiding the more open plains where it would have faced competition with the more successful and aggressive
phorusrhacids Phorusrhacids, colloquially known as terror birds, are an extinct family of large carnivorous, mostly flightless birds that were among the largest apex predators in South America during the Cenozoic era. Their definitive fossil records range from ...
and the giant vulture-like teratornithid ''
Argentavis ''Argentavis'' is an extinct genus of teratornithid known from three sites in the Epecuén and Andalhualá Formations in central and northwestern Argentina dating to the Late Miocene (Huayquerian). The type species, ''A. magnificens'', is some ...
''. Fossils of ''Thylacosmilus'' have been found in the
Huayquerian The Huayquerian () age is a period of geologic time (9.0–6.8 Ma) within the Late Miocene epoch of the Neogene, used more specifically within the SALMA classification. It follows the Chasicoan and precedes the Montehermosan age. Etymology T ...
(
Late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million ye ...
) Ituzaingó, Epecuén, and
Cerro Azul Formation The Cerro Azul Formation (), also described as Epecuén Formation, is a formation (geology), geological formation of Late Miocene (Tortonian, or Huayquerian in the South American land mammal age, SALMA classification) age in the Colorado Basin, Arg ...
s and the
Montehermosan The Montehermosan age is a period of geologic time (6.8–4.0 Mya (unit), Ma) within the Miocene and Pliocene epochs of the Neogene used more specifically with South American Land Mammal Ages. It follows the Huayquerian and precedes the Chapadmalal ...
(
Early Pliocene Early may refer to: Places in the United States * Early, Iowa, a city * Early, Texas, a city * Early Branch, a stream in Missouri * Early County, Georgia * Fort Early, Georgia, an early 19th century fort Music * Early B, stage name of Jamaican d ...
) Brochero and
Monte Hermoso Formation Monte may refer to: Places Argentina * Argentine Monte, an ecoregion * Monte Desert * Monte Partido, a ''partido'' in Buenos Aires Province Italy * Monte Bregagno * Monte Cassino * Montecorvino (disambiguation) * Montefalcione Portugal * Monte ...
s in Argentina.''Thylacosmilus''
at
Fossilworks Fossilworks was a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database, a large relational database assembled by hundreds of paleontologists from around the world. History Fossilworks was cr ...
.org


Extinction

Although older references have often stated that ''Thylacosmilus'' became extinct due to competition with the "more competitive" saber-toothed cat ''Smilodon'' during the
Great American Interchange The Great American Biotic Interchange (commonly abbreviated as GABI), also known as the Great American Interchange and the Great American Faunal Interchange, was an important late Cenozoic paleozoogeographic biotic interchange event in which land ...
, newer studies have shown this is not the case. ''Thylacosmilus'' died out during the Pliocene (3.6 to 2.58 Ma) whereas saber-toothed cats are not known from South America until the
Middle Pleistocene The Chibanian, more widely known as the Middle Pleistocene (its previous informal name), is an Age (geology), age in the international geologic timescale or a Stage (stratigraphy), stage in chronostratigraphy, being a division of the Pleistocen ...
(781,000 to 126,000 years ago). As a result, the last appearance of ''Thylacosmilus'' is separated from the first appearance of ''Smilodon'' by over one and a half million years. It’s believed changes in the landscape by the Andean uplift may have caused the extinction of ''Thylacosmilus''.


Notes


References


Bibliography

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External links

{{Taxonbar, from=Q131163 Sparassodonta Tortonian first appearances Piacenzian extinctions Neogene mammals of South America Chapadmalalan Montehermosan Huayquerian Neogene Argentina Fossils of Argentina Cerro Azul Formation Ituzaingó Formation Fossil taxa described in 1933 Taxa named by Elmer S. Riggs Prehistoric mammal genera